The present invention relates to a wind turbine generator system, and in particular to a wind turbine generator system capable of continuous operation of a wind turbine by maintaining the rotational velocity of the wind turbine.
Explanation will be given on the conventional wind turbine generator system. The wind turbine is connected to a generator. The wind turbine is rotated by wind power and the wind turbine drives the generator, so that the generator generates electricity. When using a synchronous generator as the generator, the stator of the generator is connected to a converter and AC power outputted from the generator is converted into DC power by the converter. Furthermore, the power is converted into AC power of a commercial frequency by an inverter and supplied to a power system. The converter regulates the output of the generator according to a power instruction given from outside. An example of the wind turbine generator system having such a configuration is disclosed in JP-A-2002-233193 (Paragraphs [0029] to [0031]).
The wind turbine generator system is greatly affected by fluctuation of the wind velocity, which makes the rotational velocity of the wind turbine fluctuate. When the rotational velocity of the wind turbine is out of the operation range, the operation is normally stopped so as to protect the wind turbine. Here, conventionally, when the wind velocity fluctuates, the pitch angle of the wind turbine blade is controlled according to the wind velocity or the power instruction given to the converter is adjusted according to the wind velocity, thereby suppressing the wind turbine rotational velocity fluctuation.
However, control of the pitch angle by driving the wind turbine blade according to the wind velocity includes mechanical operation and cannot have a high response. When the power instruction given to the converter is adjusted according to the wind velocity, decision is normally made by the power curve based on the average wind velocity and it is difficult to follow the transient wind velocity change. Accordingly, when the wind velocity changes suddenly and the rotational velocity of the wind turbine deviates from the operation range, the wind turbine may stop. In this case, the wind turbine should be started again.
However, in order to increase the wind turbine generating electrical quantity, it is preferable that the wind turbine continuously operate even when the wind velocity suddenly changes. When the wind turbine generating electrical quantity increases, it is possible to reduce the wind turbine generation cost. Accordingly, it is important to operate the wind turbine continuously and improve the wind turbine use ratio. Moreover, when the wind turbine can be continuously operated, the number of operation times of a contactor or the like for linking the wind turbine with a system is decreased, and hence the service life of these devices can be increased.